Radiofrequency feedthrough

ABSTRACT

A feedthrough for supporting a wire passing through the wall of a waveguide into a radiofrequency field. The body of the feedthrough is made from a lossy dielectric material and is formed so that its outer portion overlies the outer wall of the waveguide. The outer surface of the feedthrough supports a metallic film which in combination with the outer surface of the waveguide forms a radial transmission line. The dimensions of the feedthrough are such as to form a low Q resonant cavity for the resonant frequency field.

United States Patent Monaghan et a1.

[ Feb. 1, 1972 [541 RADIOFREQUENCY FEEDTHROUGH [72] Inventors: Stephen R. Monaghan, Harvard; Jerome D. l-lanfling, Framingham, both of Mass.

[73] Assignee: Raytheon Company, Lexington, Mass.

[22] Filed: June 22, 1970 [2]] Appl. No.: 48,233

[52] U5. Cl. ..333/79, 333/24.1

[51] Int. Cl. .1101]: 7/14 [58] Field oiSearch ,.333/97,81,98,24.l,79

[56] References Cited UNITED STATES PATENTS 2,994,841 8/1961 Zaleski ..333/24.l X 3,320,557 5/1967 Garstang ..333/79 3,324,426 6/1967 Brueckmann ..333l79 X Reinke et al ..333/79 Luebke ..333/79 X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Philip J. McFarland and Joseph D. Pannone [57] ABSTRACT A feedthrough for supporting a wire passing through the wall of a waveguide into a radiofrequency field. The body of thc feedthrough is made from a lossy dielectric material and is formed so that its outer portion overlies the outer wall of the waveguide. The outer surface of the feedthrough supports a metallic film which in combination with the outer surface of the waveguide forms a radial transmission line. The dimensions of the feedthrough are such as to form a low Q resonant cavity for the resonant frequency field.

5 Claims, 2 Drawing Figures RADIOFREQUENCY FEEDTI-IROUGII BACKGROUND OF THE INVENTION of individually controlled radiating elements is used to form at least one beam of radiofrequency energy.

It is known in the art that a collimated beam of radiofrequency energy may be formed and steered by controlling the phase of the energy radiated from each one of a plurality of radiating elements in an array thereof. A critical component of each element in such an array is the variable phase shifter required for collimation and steering. For satisfactory operation, variations between individual ones of such components must be minimized.

The magnitude of the problem of reducing variations between the phase shifters in an antenna array is greater when it is desired to propagate a relatively narrow beam of radiofrequency energy. For example, in an antenna array for a beamwidth of 1, approximately 10,000 radiating elements and phase shifters are required. With such a large number of phase shifters it is required that each one have no appreciable effect on the radiofrequency energy being propagated or received other than the desired effect of shifting phase. Obviously, therefore, side effects such as leakage of radiofrequency energy at the phase shifters must be eliminated or at least minimized.

When a ferrite phase shifter is used, unless extreme care is taken, small airgaps between the surface of the ferrite and the walls of the waveguide in which the ferrite is mounted may result in the excitation of higher mode radiofrequency signals in the waveguide. Such signals may be coupled to the-control wires or the phase shifter and radiated therefrom. Because the amount of higher order mode excitation and the coupling thereof to the control wires depend upon the size of the airgap between the ferrite and the walls of the waveguide, it has been customary to grind the ferrites after assembly and provide a special section of waveguide in which some sort of prestressed cover is used to eliminate the undesired airgaps. Obviously such an approach is both time consuming and expensive.

Even if the foregoing precautions are taken, it has been found that radiofrequency energy is coupled to the control wires for each such shifter, thereby causing unpredictable changes in insertion VSWR and insertion loss. In order to reduce the magnitude of such changes and the losses associated with them, it has been common practice to provide a radiofrequency attenuator, as a closely fitted sleeve of a dielectric material, for each one of the control wires of each phase shifter in an antenna array. Although the attenuation of such a sleeve dampens unwanted resonances to some extent, insertion losses are increased. Consequently, even though the radiofrequency signals being propagated in the waveguide are attenuated, relatively large radiation losses are still suffered to the detriment of proper operation of the antenna array.

Therefore, it is a primary object of this invention to provide an improved feedthrough for the control wires of a phase shifter in a microwave waveguide.

Another object of this invention is to provide an improved phase shifter which isolates the microwave energy in a microwave waveguide.

Still another object of this invention is to provide an improved feedthrough for a phase shifter in a waveguide which reduces insertion loss spikes without attenuating radiofrequency signals in the waveguide.

Still another object of this invention is to provide an improved phase shifter which may be easily and economically fabricated.

SUMMARY OF THE INVENTION These and other objects of this invention are provided generallyby providing a feedthrough for a phase shifter in a waveguide, such feedthrough being so shaped and fabricated as to constitute an insulator for each control wire passing therethrough and at the same time to constitute a low Q resonant chamber for radiofrequency energy which is coupled from the waveguide to the control wire.

I BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the invention reference is now made to the following description of the accompanying drawings in which:

FIG; 1 is a simplified sketch of a radar system in which an array of radiating elements, each connected to a ferrite phase shifter according to this invention, is space-fed to radiate a collimated beam of radiofrequency energy and to receive echo signals from targets illuminated by such radiated energy; and

FIG. 2 is a partial cross section of the ferrite phase shifter shown in FIG. 1, such partial cross section being taken to illustrate in detail the design and fabrication of the feedthrough contemplated by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 it may be seen that an antenna 10 according to this invention includes a number of phase shifters 11 (here of the digital latching type) each having associated with it a driver 13. The phase shifters and drivers may be mounted in any conventional manner (not shown in detail) to form aspace-fed planar array. Appropriate connections are made asliindicated between each driver 13 and the conductor for each ferrite toroid (shown in FIG. 2) of each phase shifter 11 to set and reset" each phase shifter 11 in accordance with control signals from a beam steering computer 15. As is known, radiofrequency energy from a feed horn 17 may so be collimated and directed in a beam as desired and echo signals returning to the individual phase shifters of the antenna array may be focused on the feed horn 17. The feed horn 17 is connected in any convenient manner by waveguide (not numbered) to a transmitter/receiver 19. The operation of the latter element and the beam steering computer 15 is controlled by a conventional synchronizer 21.

Each one of the phase shifters 11 includes a section of waveguide 23 with the ends thereof matched to free space by conventional matching devices 25, 25. In the particular embodiment illustrated, each one of the phase shifters II includes three serially arranged ferrite toroids 26a, 26b, 260 (FIG. 2) to provide for a three-bit control signal. Two control wires 27r, 27r (FIG. 2) are provided for each one of the ferrite toroids 26a, 26b, 260, one of such wiresbeing energized during the transmit portion and the other one being energized during the receive portion of each radar sweep as required when a nonreciprocal latching phase shifter is used. As is more clearly shown in FIG. 2, each one of the control wires 26!, 26r for each one of the ferrite toroids 26a, 26b, 26c is led through a feedthrough 30 mounted in a wall of the Waveguide 23 to thread through its associated ferrite toroid 26a, 26b, 26c. Referring now to FIG. 2 the details of construction of each feedthrough 30 and the manner in which each such feedthrough is mounted may be seen. Thus, the body of each feedthrough 30 is a molded cap-shaped member 31, or plug, of a dielectric material, as equivalent to MF-1l7, (an iron loaded epoxy), manufactured by Emerson-Cummings, Canton, Mass. mounted in any convenient manner, as by cementing (not shown), to a wall of the waveguide 23. A control wire 27r, 27r is led centrally of the body of the feedthrough 30 as indicated. The free surface of the body of the feedthrough 31 is covered, as by plating, with a metallic film 32, each control wire 27r, 27r being connected, as by solder, thereto as indicated. A dielectric separator 33, as magnesium titanate, having its ends formed to accommodate the control wires as shown, may be included to complete the illustrated phase shifter 11. The entire assembly may be secured in the waveguide 23 by cementing (not shown).

The dimensions of the feedthrough 30 are dependent upon the frequency of the radiofrequency energy propagated through the waveguide 23 so that the feedthrough acts as a resonant radial transmission line for such energy. The metallic film 32 and the underlying portion of the molded cap-shaped member 31 are of such diameter that a low impedance appears at the plane where such wire passes through the inner surface of the waveguide 23. In other words, such diameter is equal to one-quarter of the wavelength of the radiofrequency energy being propagated through the waveguide. At the same time, the spacing between the outer surface of the waveguide 23 and the metallic film 32 is made as small as is convenient so that the impedance of the radial line formed by such elements is high at the open end of such line without weakening the physical strength of the structure too much. By making the impedance at the open end of the radial line high, an impedance mismatch exists between the feedthrough and free space, thereby increasing the isolation between the radial line and free space. The higher the isolation the less radiofrequency energy is radiated. Even if higher order modes are excited in the waveguide 23 such modes will be damped out in the feedthrough 30 if the loss tangent of the molded cap-shaped member 31 is large. in such a case the feedthrough 30 and the ferrite toroids 26a, 26b, 26c become the predominant lossy elements ofa resonant circuit with high loss, thereby reducing the Q of the feedthrough to eliminate insertion loss spikes. in a practical embodiment of this invention where the wavelength of the radiofrequency energy in the waveguide 23 is 1.73 inches, the spacing between the metallic film 32 and the outside of the waveguide 23 may be in the order of 0.010 inches and the diameter of the metallic film 32 may be in the order of 0.350 inches to 0.700 inches. Thus, when the loss tangent of the dielectric material making up the molded cap-shaped member 31 is in the order of 0.1, a VSWR between 500 and 1,000 occurs. The corresponding isolation with such a structure is between 46 and 49 db.

in view of the foregoing it will be apparent that changes in the disclosed embodiment of the invention may be made without departing from our inventive concepts. For example, the dielectric material in the feedthrough may be changed, provided only that a material which has a relatively high loss tangent for the radiofrequency signals being propagated is used. Further, the disclosed feedthrough may be used with types of radiofrequency transmission lines other than waveguide and with components other than digital latching ferrite phase shifters. it is also obvious that the dimensions and shape of the disclosed feedthrough may be changed for different shapes of transmission lines and different frequencies. That is, it is necessary only to follow the concept of providing a combination of an electrical insulator for control wires and a low Q resonant structure for radiofrequency signals to eliminate both radiation and resonance losses by establishing a low impedance at the junction of the transmission line and the feedthrough (to eliminate radiation losses) and by providing an attenuating path within the feedthrough (to dampen resonance peaks). Thus, even though the radial line embodiment shown and described hereinbefore is preferred because such embodiment projects the least amount from the wall of the waveguide, a straight sleeve of proper length with an electrically conductive coating could be used.

It is felt, therefore, that this invention should not be restricted to the proposed embodiment, but rather should be limited only by the spirit and scope ofthe following claims.

What is claimed is:

1. A feedthrough for a wire passing through a circular opening formed in a wall ofa shielded radiofrequency transmission line and coupling with a field of radiofrequency energy within such line, such feedthrough comprising:

a. an electrically conductive disclike member affixed orthogonally to the wire, the curvature of such member corresponding to the curvature of the outside of the shielded radiofrequency transmission line and the radius of such member being an odd integral multiple of onequarter wavelength of the radiofrequency energy within the shielded radiofre uency transmission line; and, b. mounting means fort e wire and the electrically conductive disclike member affixed thereto, such means cooperating between at least the wire and the wall of the shielded radiofrequency transmission line to electrically insulate the wire and the electrically conductive disclike member from the wall of the shielded radiofrequency transmission line and to form a radial transmission line between such member and such wall.

2. A feedthrough as in claim 1 wherein the mounting means includes a solid dielectric plug having at least a first portion, the curvature and radius of such plug corresponding, respectively, to the curvature and radius of the electrically conductive disclike member and the thickness of such first portion corresponding to the spacing between such member and the wall ofthe shielded radiofrequency transmission line.

3. A feedthrough as in claim 2 wherein the thickness of the at least first portion of the solid dielectric plug is less than the diameter of the opening in the wall of the shielded radiofrequency transmission line.

4. A feedthrough as in claim 3 wherein the solid dielectric plug includes a second portion, such second portion having a diameter substantially equal to the diameter of the opening in the wall of the shielded radiofrequency transmission line and a thickness substantially equal to the thickness of such wall.

5. A feedthrough as in claim 4 wherein the loss tangent of the material of the solid dielectric plug is greater than 0. l.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3.639 .863 DatdM lnventofls) Stephen R. Monaghan and Jerome D. Hanfling It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, after the title "RADIO FREQUENCY FEEDTHROUGH" insert The invention herein described was made in the course of or under a contract or subcontract thereunder, with theDepartment of Defense.

Signed and sealed this 1st day of August 1972;

(SEAL) Attest:

- EDWARD M.FLE TCHER,JR. ROBERT GUTTSCHALK Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 u.s. GOVERNMENT PRINTING OFFICE I969 crass-3:

FORM PO-105O (10-69) 

1. A feedthrough for a wire passing through a circular opening formed in a wall of a shielded radiofrequency transmission line and coupling with a field of radiofrequency energy within such line, such feedthrough comprising: a. an electrically conductive disclike member affixed orthogonally to the wire, the curvature of such member corresponding to the curvature of the outside of the shielded radiofrequency transmission line and the radius of such member being an odd integral multiple of one-quarter wavelength of the radiofrequency energy within the shielded radiofrequency transmission line; and, b. mounting means for the wire and the electrically conductive disclike member affixed thereto, such means cooperating between at least the wire and the wall of the shielded radiofrequency transmission line to electrically insulate the wire and the electrically conductive disclike member from the wall of the shielded radiofrequency transmission line and to form a radial transmission line between such member and such wall.
 2. A feedthrough as in claim 1 wherein the mounting means includes a solid dielectric plug having at least a first portion, the curvature and radius of such plug corresponding, respectively, to the curvature and radius of the electrically conductive disclike member and the thickness of such first portion corresponding to the spacing between such member and the wall of the shielded radiofrequency transmission line.
 3. A feedthrough as in claim 2 wherein the thickness of the at least first portion of the solid dielectric plug is less than the diameter of the opening in the wall of the shielded radiofrequency transmission line.
 4. A feedthrough as in claim 3 wherein the solid dielectric plug includes a second portion, such second portion having a diameter substantially equal to the diameter of the opening in the wall of the shielded radiofrequency transmission line and a thickness substantially equal to the thickness of such wall.
 5. A feedthrough as in claim 4 wherein the loss tangent of the material of the solid dielectric plug is greater than 0.1. 